In turning of a metal work piece, the metal work piece rotates around a center axis. The metal work piece is clamped at one end by rotatable clamping means such as one or more chuck or jaws. The end of the work piece which is clamped can be called a clamping end or a driving end. For stable clamping, the clamping end or the driving end of the metal work piece may have a larger diameter than the opposite end of the metal work piece and/or has a larger diameter of a portion of the metal work piece located between the clamping end and the opposite end. Alternatively, the metal work piece may have a constant diameter before a machining, i.e. metal cutting, operation.
The turning insert is moved in relation to the metal work piece. This relative movement is called feed. The movement of the turning insert can be in a direction parallel to the center axis of the metal work piece, this is commonly called longitudinal feed or axial feed. The movement of turning insert can furthermore be in a direction perpendicular to the center axis of the metal work piece, this is commonly called radial feed or facing. Other angles of movement, or feed directions, are also possible, this is commonly known as copying or copy-turning.
In copying, the feed has both axial and radial components. During the relative movement of the turning insert, material from the metal work piece is removed in the form of chips. The chips are may be short and/or have a shape or direction of movement which prevents chip jamming and/or do not give a poor surface finish of the machined surface.
Common shapes of turning inserts which can be used for a wide range of feed direction include triangular turning inserts. Such inserts have in a top view, i.e. a rake face towards the viewer, the shape of a triangle where all three sides are of equal length and where the nose angle is 60°. The corners of the triangle are in the form of nose cutting edges, which typically has a radius of curvature in the range of 0.2-2.0 mm. Examples of such turning inserts are commonly designated TNMG and TCMT according to ISO standard, and are commonly made at least partly from coated or uncoated cemented carbide or cubic boron nitride (CBN) or ceramic or cermet.
Other common shapes of turning inserts have in a top view, i.e. a rake face towards the viewer, the shape of a rhombus where all four sides are of equal length and where the nose angle of an active nose portion is 80°. The active corners are in the form of nose cutting edges, which typically has a radius of curvature in the range of 0.2-2.0 mm. Examples of such turning inserts are commonly designated CNMG according to ISO standard, and are commonly made at least partly from coated or uncoated cemented carbide or cubic boron nitride (CBN) or ceramic or cermet.
Both the described triangular and rhombic turning inserts can be used for turning two walls forming an external 90° corner in a metal work piece, where one wall, at a greater distance from the rotational axis of a metal work piece, is perpendicular to the rotational axis and one cylindrical wall, at a smaller distance from the rotational axis, is parallel to the rotational axis, where the two walls are connected by a circular or curved segment. An external 90° corner in this context is a 90° corner formed on or at an external or outer surface of a metal work piece, such that the cylindrical wall or cylindrical surface is facing away from the rotational axis. This is in contrast to any corner which may be formed on or at an internal or inner surface inside a bore concentric with the rotational axis.
The circular or curved segment have a cross-section in a plane including the rotational axis in the shape of an arc, in the shape of a quarter of a circle or a quarter of a shape, which is substantially a circle, which has the same radius of curvature as the nose cutting edge of the turning insert. The circular or curved segment alternatively has a greater radius of curvature than the nose cutting edge of the turning insert.
In EP2572816B1, a turning tool is shown during machining of a work piece. The turning tool can be used for forming two walls forming an external 90° corner, without any reorientation of the turning tool. The tool includes a holder as well as a turning insert. In this case, the work piece is rotated at the same time as the tool is longitudinally fed parallel to the center axis of the. The setting angle, or entering angle, is the angle between the direction of the longitudinal feed and a main edge. The setting angle, or entering angle, is 95°. The turning insert has a rhombic basic shape and comprises two acute-angled corners having an angle of 80° and two obtuse-angled ones having an angle of 100°. A back clearance angle of 5° is obtained between the turning insert and the generated surface of the work piece. The generated surface of the work piece is substantially cylindrical.
Such a turning method as in EP2572816B1 gives an unsatisfactory tool life, or usage time, for the turning insert.